At present, most high speed (1 Gbps or more) electrical communication over cables is done through differential cables. Differential communication is considered as the best way to achieve sufficient Electro Magnetic Compatibility (EMC), being low radiative emissions at the transmit side and good bulk current immunity at the receive side. Many examples of commercialized technologies exist, like USB (Universal Serial Bus), HDMI (High Definition Multimedia Interface), DisplayPort, Serial ATA (Advanced Technology Attachment), etc . . . based on differential signaling. However, differential connectors and cables are more expensive than single ended coaxial and non-coaxial cables and connectors. Furthermore, at high bit rate, differential cables often generate intra-pair skew. This leads to wave coupling in the cable between differential and common modes, and to final destruction of the transmitted signals. Skew compensation circuits can only compensate very limited skew at receiver's end due to the destructive effect of the coupling. A single ended cable like a coaxial cable (or coax) cannot have any skew problems due to its single ended nature.
Another merit of single ended cabling systems is their relatively well-known characteristic impedance. E.g. one can easily find on the market an RG174 coaxial cable with a 50Ω impedance having a tolerance of +/−2Ω, whilst for a differential system, a typical specification is 100Ω impedance with a tolerance of +/−10Ω. This higher relative uncertainty on the characteristic impedance makes a differential cable a less attractive candidate for bidirectional communication.
U.S. Pat. No. 6,426,970B1 shows an active bidirectional splitter for communication over a common coaxial cable and also many other prior art circuits using transformers and chokes. It does not show how to get enough splitter separation or how to integrate a cable equalizer without these magnetic elements. At high bit rate, e.g. over 2 Gbps, magnetic components are hard to make, and thus expensive. On-chip transformers and/or common mode chokes achieve high quality at many GigaHertz, however they do not show wideband operation due to the limited numbers of turns that are achievable on-chip. As such they do not provide a operable solution for many types of signal communication, including NRZ (non-return to zero) and PAM (pulse amplitude modulation) communication. Having a high level of splitter separation, allows to use bidirectional communication including equalizer receiver functions for compensating frequency dependent losses in the cable. In this way, bidirectional communication over a single coaxial cable becomes possible in a full duplex mode using extended cable lengths and/or at higher bitrates.
US2003/123570 describes a receiver capable of receiving signals in simultaneous bi-directional current mode differential links. The receiver comprises a resistor-summing network coupled to the outputs of a data driver and to the outputs of a replica driver. One portion of the resistor network outputs an average voltage of the positive phase of the reference driver and the negative phase of the data driver. A second portion of the resistor-summing network outputs an average voltage of the negative phase of the reference driver and the positive phase of the data driver. The two outputs of the resistor network are coupled to the inputs of a differential amplifier. The output of the amplifier is the data sent by the remote data driver. It is a disadvantage of such receiver that common mode signals are not cancelled out.
US2002/149402 describes a simultaneous bidirections data port circuit including a current mode output driver for driving an output node and a current mode return driver for driving a differential receiver. Each driver is divided into driver segments. Some driver segments are driven by outbound data, and other driver segments are driven by pre-equalization data.